1. Field of the Invention
The invention relates generally to spread spectrum mobile communication networks, and more particularly, to a method and apparatus for generating an RF signal from a baseband signal for communication in a spread spectrum mobile cellular communication network.
2. Description of Related Art
In wireless radiotelephone communication systems, several users may communicate over one or more wireless channels. Communication over the wireless channels may employ a variety of multiple access techniques that enable multiple users to share limited frequency spectrum. These multiple access techniques may include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA).
An exemplary CDMA system is described in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,” issued Feb. 13, 1990, incorporated herein by reference. An exemplary CDMA system is further described in U.S. Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM,” issued Apr. 7, 1992, and incorporated herein by reference.
As described in the above referenced patents, CDMA systems employ a forward-link (base station to mobile station) pilot signal. In typical CDMA wireless communication systems, such as that described in EIA/TIA IS-95, the pilot signal acts as a “beacon” transmitting a constant zero symbol, spread with the same pseudo-noise (PN) sequences used by the traffic bearing signals. The pilot signal may be covered with an all-zero Walsh sequence. During initial system acquisition, a mobile station may search through PN offsets to locate a base station's pilot signal. Once the mobile station has acquired the pilot signal, it may derive a stable phase and magnitude reference for coherent demodulation and modulation (transmission to the base station), such as that described in U.S. Pat. No. 5,764,687 entitled “MOBILE DEMODULATOR ARCHITECTURE FOR A SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM,” issued Jun. 9, 1998, and incorporated herein by reference.
In addition, mobile units may be required to transmit signals using one or more RF frequencies and related channels. In such networks mobile units generally have limited transmit power. In order to conserve power and prevent or reduce dropped calls each mobile unit should transmit precisely modulated signals (with stable phase and magnitude) to a base station. The present invention provides such a mobile unit based system transmitter.